Hypergolic ignition method using organoaluminum compositions



United States Patent Ofice 3,524,317 Patented Aug. 18, 1970 3,524,317 HYPERGOLIC IGNITION METHOD USING ORGANOALUMINUM COMPOSITIONS Martin E. Gluckstein, Detroit, Mich., assignor to Ethyl Corporation, New York, N .Y., a corporation of Virginia No Drawing. Filed Dec. 22, 1967, Ser. No. 692,704

Int. Cl. C0611 5/10 US. Cl. 60-209 16 Claims ABSTRACT OF THE DISCLOSURE A pyrophoric fuel capable of spontaneous ignition which consists essentially of a mixture of organoaluminum compounds, provided that at least one compound in the mixture is an organoaluminum hydride or halide and the the amount of each compound in the mixture is at least ten weight percent. A method of effecting combustion comprises contacting said fuel with air or other oxidizer.

BACKGROUND OF THE INVENTION v Fuels presently available are generally hydrocarbon fuels. However, these fuels generally do not posses spontaneous ignition characteristics. Furthermore, since such fuels normally are not pyrophoric, they require an igniter system to effect their ignition. Thus, the ignition delay of a hydrocarbon fuel depends not only on the inherent ignition delay of the fuel itself, but also on any mechanical delay inherent in the ignition system. Because of the presence of an ignition system, the possibility of failure is increased. In contrast, the fuels of this invention provide fuels which are pyrophoric and, therefore, will ignite on contact with an oxidizing agent, thus eliminating any need for an igniter system and the disadvantages associated therewith.

Fuels based on organometallic compounds have undergone considerable research, and their advantages have been recognized in the art. For example, the Bauerle et al. patent, US. 3,127,735, deals with reduced ignitiondelay fuels prepared from organometallic compounds and organoboranes. More specifically, said patent discloses fuels (propellants) composed of a variety of organometallic compounds, including aluminum alkyls and alkyl boron compounds, such as trialkyl boranes, alkyl boron halides, and alkyl diboranes. To function as a fuel, the propellant must react with a liquid oxidizer such as liquid oxygen, nitrogen tetroxide, hydrogen peroxide, liquid fluorine, fuming nitric acid, and the like.

The fuels of this invention vary substantially from the fuels described in the above-mentioned Bauerle et al. patent. In the instant invention, the fuels com-prise a mixture of organoaluminum compounds, at least one of which is an organoaluminum hydride or halide. Another important feature of this invention is that the fuels do not require a strong oxidizing agent, such as liquid oxygen, hydrogen peroxide and the like, but are fully operational when air is used as the oxidizer, even at such low pressures as 0.25 atmosphere.

SUMMARY OF THE INVENTION DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention comprises the discovery of fuels with extremely short ignition delays. These fuels can be provided 'by a composition consisting essentially of a mixture of different organoaluminum compounds as defined below provided that at least one organoaluminum compound is an organoaluminum hydride or an organoaluminum halide. The compounds employed in the mixture are present in the amount of at least 10 weight percent.

The preferred fuel compositions comprise mixtures of .trialkylaluminum and dialkylaluminum halide compounds, trialkylaluminum and dialkylaluminum hydride compounds, dialkylaluminum halide and dialkylaluminum hydride compounds, and trialkylaluminum, dialkylaluminum hydride, and dialkylaluminum halide compounds.

giljhe most preferred fuels comprise substantially the following mixtures:

(is) From about 15 to about 60 weight percent trimethylaluminum and from about 40 to about 85 weight percent of a dialkylaluminum halide or hydride;

(13) From about 10 to about 60 weight percent of a triallrylaluminum and from about 40 to 90 weight percent of. a dialkylaluminum hydride;

(c) From about 35 to about 70 weight percent of a trialkylaluminum and from about 30 to about 65 weight perbent of a dialkylaluminum halide; and

id) From about 10 to about 60 weight percent of a dialkylaluminum halide and from about 40 to about 90 weight percent of a dialkylaluminum hydride.

- .When contacted with an oxidizer, the fuels of this invention ignite spontaneously. By spontaneously, it is meant that the ignition takes place within one second, and preferably, within a much shorter interval of time. The instant fuels ignite generally within about 25 milliseconds. However, the ignition delay time for the preferred compositions of this invention is from about 10 to about 20 milliseconds. When the recorded ignition delay time is corrected for the discrepancy inherent in the recorder, as

" explained below, the true ignition delay of the most preferred compositions drops to about 5 to 15 milliseconds aft'er contact.

The components of the fuels of this invention are organoaluminum compounds having the empirical formula R Al wherein the R groups are selected from hydrocarbon radicals such as alkyl groups having up to about 4 carbon atoms or aryl, aralkyl and alkaryl groups having up to about 8 carbon atoms, hydrogen, and halogens, especially chlorine, bromine and fluorine. It should be noted, however, that the organoaluminum compounds should not have more than one aryl, alkaryl or aralkyl group. Although in some instances iodine and more than one aromatic group may be present in the organoaluminum compounds, generally, such compounds are not very pyrophoric and, therefore, preferably should not be employed in the fuels of this invention.

A further requirement with regard to organoaluminum compounds is that the compounds should have at least one hydrocarbon group present. In other words, any given aluminum compound cannot have more than two hydrogen and/ or halogen atoms present. The hydrocarbon groups which are bonded to aluminum may be halogenated with chlorine, bromine, and fluorine, and in limited instances, also with iodine. The three R groups bonded to aluminum may be the same or different, keeping in mind the above noted requirements.

The alkyl groups attached to aluminum may be either straight chain or branched chain containing preferably up to four carbon atoms. Non-limiting examples of alkyl aluminum compounds, that is, compounds whose carbon groups areselected only from alkyl radicals, are trimethylalrirninum, triethylaluminum, tri-n-propylaluminum, trin-butylaluminum, triisobutylaluminum, triisopropylaluminum, triisobutylaluminum hydride, diethylaluminum hydride, di-n-propylaluminum hydride, diethylaluminum chloride, ethylaluminum dichloride, methylaluminum sesquichloride, ethylaluminum sesquichloride, diisobutylaluminum chloride, methylaluminum sesquibromide, diethylaluminum iodide, methyldiethylaluminum, ethyldi-nbutylaluminum, dimethylisobutylaluminum, diethylmethylaluminum, and the like. It should be noted that some organoaluminum compounds exist as dimers and trimers. For example, trimethylaluminum, triethylaluminum, and tri-n-isopropylaluminum exist as dimers, while dialkylaluminum hydrides exist as trimers.

The organoaluminum compounds useful in the fuels of this invention may contain one aryl, aralkyl, or alkaryl group. Thus, a phenyl group may be attached to aluminum producing such compounds as dimethylphenylaluminum, diethylphenylaluminum, diisopropylphenylaluminum, ethylphenylaluminum hydride, and the like."Although polyaryl and alkarylaluminum compounds could also be employed, such compounds have low pyrophoric properties and, therefore, the resulting fuels would have less desirable ignition properties.

The alkaryl groups which may be attached to aluminum contain a single aromatic ring with from 1 to 3 lower alkyl substituents such as methyl, ethyl, and propyl. Nonlimiting examples are dimethyltolylalurninum, diethyltolylaluminum, dimethyl(2,4-diethylphenyl)aluminum, (3- isopropylphenyl)methylaluminum hydride, (4-chloro-2- methylphenyl)methylaluminum hydride, and the like.

The aralkyl groups which may be attached to aluminum may contain up to three carbon atoms in the alkyl chain and a single aromatic ring substituted with one to three lower alkyl substituents. Non-limiting examples are benzyldimethylaluminum, benzyldiethylaluminum, dimethyl (4-methylbenzyl aluminum, (4-chloro-2-isopropylbenzyl)ethylaluminum hydride, methyl(2,3,4-trime'thylbenzyl)aluminum hydride, [3-(4-isopropylphenyl)propyl] v tionation usually takes place. This means that the various organic groups, hydrogen and/or halogen radicals, interchange until a dynamic equilibrium is reached. The end result is that the composition is not merely a mixture of two or more compounds, but a mixture of many compounds containing compositions intermediate between the original compounds.

The operation of disproportionation may be illustrated by the following example. If originally triethylaluminum, triisobutylaluminum, and diethylaluminum hydride are mixed, the resulting disproportionation product will contain the following compounds: diethylisobntylaluminum, ethylisobutylaluminum hydride, ethylisobutylaluminum, and diisobutylaluminum hydride, in addition to the original compounds. If such compounds as dialkylaluminum halide, R AlX, and dialkylaluminum hydride, R AlH, are mixed, the redistribution product would probably contain, in addition to the original compounds, the following intermediate compounds: RAlX RAIH and RAlXI-I.

The important feature of this invention is that the ignition delay, defined as the time elapsed from the moment of contact of the fuel with an oxidizer to the moment of ignition, is lower than the predicted ignition delay. The predicted ignition delay values for two component fuels were obtained by plotting on a graph (where one axis represents the ignition delay time while the other represents the fuel composition) the ignition delay for each component of the fuel. Thus, one end of the composition axis represents 100 percent of one component while the other end represents 100 percent of the other component. The line in between represents fuel compositions containing a certain amount of both components. When a straight line is drawn between the two points representing the ignition delays of the pure components, one can obtain from the graph a predicted interpolated ignition delay value depending on the relative amounts of the two components.

The ignition delay determinations were measured by noting the time between the contacting of the fuel with an oxidizer and the time of ignition. This was accomplished in an apparatus comprising a glass combustion tube, a solenoid operated injector, and timing circuits. When the starter was depressed, the solenoid activated a fuel ejector injecting a sample of the fuel into the com bustion tube and, at the same time, activating an electronic timer. When the fuel ignited, the light was detected by a photocell which stopped the timer. It should be noted that the above-described apparatus did not give absolute ignition delay time but only relative values.

' When the ignition delay was studied by a high-speed mo tion picture camera, it was determined that in order to obtain absolute ignition delay times, about 3.5 to 4.5 milliseconds should be subtracted from the ignition delay values obtained by the above-described apparatus.

Table I below illustrates the ignition delay characteristics of the fuels of this invention. In the table, the actual and the predicted interpolated ignition delay values for the indicated compositions are reported. The actual values are uncorrected.

TABLE I Fuel components Predicted igni- Actual ignition tion delay in delay in milli- Wt. percent Wt. percent milliseconds seconds Triethyl- Diethylalumialuminum num hydride Diethylalumialuminum num hydride Diethylalurninum Diethylalurnichloride num hydride Trlethylaluml- Diethylaluminum num chloride Trimethylalumi- Diethylaluminum num chloride From the above table it is seen that a great reduction in the ignition delay characteristics of aluminum alkyls is obtained when mixtures of organoaluminum compounds are used. This result is most unexpected and startling because the various organoaluminum compounds per se possess higher ignition delay properties. Thus, it was found that by admixing organoaluminum compounds having considerable ignition delays, a fuel having greatly reduced ignition delay is obtained.

Additional examples of fuel compositions with short ignition delays which are within the scope of this invention are listed below.

Fuel composition: Weight percent Triisobutylaluminum Diethylaluminum hydride 90 Trimethylaluminum 60 Ethylphenylaluminum hydride 40 (4-chloro-2-methylphenyl)dimethylaluminum 20 Dimethylaluminum hydride 80 Trimethylaluminum 25 Triisopropylaluminum (4 chloro 2-isopropylbenzyl)methylalummum hydride 60 Benzyldiethylaluminum 30 Dimethylaluminum hydride 45 Ethylphenylaluminum hydride 25 Dimethyl(2,3,4-trimethylbenzyl)aluminum 35 v Ethylaluminum dihydride 65 Trimethylaluminum 15 Dimethylaluminum chloride 85 Trimethylaluminum 60 Diethylalumm' um bromide 40 Trimethylaluminum 15 Diisopropylaluminum fluoride 85 Trimethylaluminum 45 Ethylaluminum sesquichloride 55 Trimethylaluminum 60 Diethylaluminum iodide 40 Trimethylaluminum n-Butylaluminum dichloride 80 Trimethylaluminum 30 Methylaluminum sesquichloride 45 Ethylphenylaluminum bromide Trimethylaluminum 35 Propylaluminum difluoride 45 (3-ethylphenyl)methylaluminum bromide 20 Trimethylaluminum -Q 15 Diethylaluminum hydride 85 Trimethylaluminum 60 Diisopropylaluminum hydride 40 Trimethylaluminum 25 Ethylphenylaluminum hydride 75 Trimethylaluminum 55 Benzylmethylaluminum hydride 45 Trimethylaluminum 35 n-Butylaluminum dihydride 65 Trimethylaluminum 40 (4 chloro 2-isopropylbenzyl)methylaluminum hydride 35 Dimethylaluminum hydride 25 Trimethylaluminum 45 Ethylphenylaluminum hydride 40 Ethylaluminum dihydride 15 Trimethylaluminum 35 Diethylaluminnm bromide 65 Triethylaluminum 70 Dimethylaluminum chloride Triisobutylaluminum 40 Diisopropylaluminum fluoride 60 (4-bromo-5-ethylphenyl)dimethylaluminum 60 Ethylaluminum sesquichloride 40 Ethyl(2,4-dimethylphenyl)aluminum hydride 40 Dimethylaluminum fluoride 60 Ethylaluminum dihydride 30 Phenylaluminum dichloride 70 Benzylethylaluminum hydride 35 Methylaluminum dibromide 65 (4 chloro 2-isopropylbenzyl)methylaluminum hydride 30 Methylaluminum sesquibromide 40 Ethylpropylaluminum chloride 30 Ethylmethylaluminum hydride 25 2,4-dimethylphenylaluminum dihydride 15 Benzylmethylaluminum chloride 60 The fuels of this invention are pyrophoric. This is one of the important features and advantages of the instant fuels because it makes possible for the fuels to be employed without an igniter or special oxidizer as long as there is air. A further feature of this invention is that the fuels are operable under conditions where the pressure of air in the vicinity of the point of injection of the fuel is as low as 0.5 atmosphere, or even 0.25 atmosphere. Although the instant fuels operate satisfactorily in air containing any amount of water vapor, superior performance is obtained when air contains at least 0.00020 pound of water per cubic foot of air at atmospheric pressure.

In addition to air, other gaseous or liquid oxidizers may beused. Examples of additional oxidizers are oxygen, nitrogen tetroxide, hydrogen peroxide, chlorine trifluorine, bromine pentafluorine, white fuming nitric acid, red fuming nitric acid, liquid fluorine, liquid fluorine and liquid oxygen mixtures, oxyhalides, nitrogen fluoride, as well as other oxidizers known to those skilled in the art.

With any of the above-mentioned oxidizing agents, combustion may be either of the external or internal types. That is, combustion may take place on the external surface of a vehicle, or in a motor, such as ramjet or a rocket. When air is employed as the oxidizing agent, combustion preferably takes place on the external surface. However, when other gaseous or liquid oxidizers are employed, then an internal type combustion is preferred.

The fuels of this invention generally may be employed in all applications where a high energy propellant is required. For example, the instant fuels may be used as pyrophoric rocket propellants. The oxidizer may be either air or other oxidizing agent, depending whether the rocket is operated within the atmosphere or outside. The fuels may also be used advantageously in turbine engines to re-ignite the fuel normally employed in case of a flameout. Another important application for these fuels is as missile outboard ejector fuels. Such fuels must possess an extremely short ignition delay time and, for this reason, the fuels of this invention are particularly suitable for this application.

As stated above, organoaluminum compounds undergo disproportionation when two or more such compounds are mixed. However, for simplicity, the compositions of this invention claimed and exemplified herein are not defined in terms of redistribution products, but only in terms of the organoaluminum compounds added to form the composition. For practical reasons, the instant compositions contain one to three added organoaluminum compounds. However, these compositions may also contain four, five, or even nine organoaluminum compounds and similarly, more than three boron hydrides.

Having fully disclosed the novel fuels of this invention and the method of effecting a spontaneous ignition, it is desired that this invention be limited only within the lawful scope of the appended claims.

I claim:

1. A high energy pyrophoric fuel composition capable of spontaneous ignition, said fuel composition consisting essentially of a mixture of organoaluminum compounds selected from the group consisting of (a) mixtures of from about 15 to about 60 weight percent of at least one trialkylaluminum compound and from about 40 to about 85 weight percent of at least one dialkylaluminum chloride compound;

(b) mixtures of from about 10 to about 60 weight percent of at least one trialkylaluminum and from about 40 to about 90 weight percent of at least one dialkylaluminum hydride compound; and

(c) mixtures of from about 10 to about 60 weight percent of at least one dialkylaluminum chloride and from about 40 to about 90 weight percent of at least one dialkylaluminum hydride compound; said organoaluminum compounds having the formula R Al wherein the R groups are independently selected from hydrogen, chlorine and lower alkyl hydrocarbon radicals containing up to four carbon atoms.

2. A fuel composition of claim 1, said mixture consisting essentially of from about to about 45 weight percent of trimethylaluminum and from about 55 to about 85 weight percent of an alkylaluminum halide.

3. A fuel composition of claim 2 wherein the mixture consists essentially of about percent trimethylaluminum and 80 percent dimethylaluminum chloride.

4. A fuel composition of claim 1, said mixture consisting essentially of from about 15 to about 45 weight percent trimethylaluminum and from about 55 to about 85 weight percent of an alkylaluminum hydride.

5. A fuel composition of claim 4 wherein the mixture consists essentially of about 50 weight percent trimethylaluminum and 50 weight percent diethylaluminum hydride.

6. A fuel composition of claim 1, said mixture consisting essentially of from about 10 to about 60 weight percent of a trialkylaluminum and from about 40 to 90 weight percent of an alkylaluminum hydride.

7. A fuel composition of claim 6 wherein the mixture consists essentially of from about 20 to about 40 weight percent triethylaluminum and from about 60 to about 80 weight percent diethylaluminum hydride.

8. A fuel composition of claim 1, said mixture consisting essentially of from about to about 70 weight percent of a trialkylaluminum and from about 30 to about 65 weight percent of an alkylaluminum halide.

9. A fuel composition of claim 8 wherein the mixture consists essentially of from about to about 60 weight percent of trimethylaluminum and from about 40 to 60 weight percent of dimethylaluminum chloride.

10. A fuel composition of claim 8 wherein the mixture consists essentially of about weight percent triethylaluminum and about weight percent diethylaluminum chloride.

11. A fuel composition of claim 1, said mixture consisting essentially of from about 10 to about Weight percent of an alkylaluminum halide and from about 40 to about 90 weight percent of an alkylaluminum hydride.

12. A fuel composition of claim 11 wherein the mixture consists essentially of from about 20 to about 40 weight percent of diethylaluminum chloride and from about 60 to about 80 weight percent of diethylaluminum hydride.

13. A fuel composition of claim 12 wherein the mixture consists essentially of about 30 weight percent diethylaluminum chloride and about weight percent diethylaluminum hydride.

14.- A method of effecting spontaneous ignition, the method comprising contacting a fuel composition of claim 1 with an oxidizer.

15. A method of claim 14 wherein said oxidizer is air.

16. A method of claim 15 wherein said air contains at least 0.00020 pound of water per cubic foot of air at atmospheric pressure.

References Cited UNITED STATES PATENTS 

